CO2 recovery unit and CO2 recovery method

ABSTRACT

A CO 2  recovery unit includes a CO 2 -absorber that causes a gas containing CO 2  to contact a CO 2 -absorbing solution and that causes the CO 2  in the gas to be absorbed into the CO 2 -absorbing solution; a CO 2 -regenerator that heats the CO 2 -absorbing solution, releases the CO 2  from the CO 2 -absorbing solution, and regenerates the CO 2 -absorbing solution; and a CO 2  recovery amount controller that: calculates a computed target value of a CO 2  recovery amount and a computed target value of a CO 2  recovery rate based on a set value of the CO 2  recovery rate, actual measured values of CO 2  concentration, gas flow rate, and temperature of the gas, and calculates a maximum value of the CO 2  recovery amount in the CO 2 -absorber and a maximum value of the CO 2  recovery amount in the CO 2 -regenerator.

TECHNICAL FIELD

The present invention relates to a CO₂ recovery unit and a CO₂ recoverymethod, for example, a CO₂ recovery unit and a CO₂ recovery method thatrecover CO₂ in a gas to be treated, using a CO₂-absorbing solution.

BACKGROUND

In the related art, CO₂ recovery units that recover CO₂ exhausted fromboilers or the like of thermoelectric power plants are suggested (forexample, refer to PTL 1). In the CO₂ recovery units, flue gas isintroduced into a CO₂-absorber, a CO₂-absorbing solution is brought intocontact with CO₂ included in the flue gas so that CO₂ is made to beabsorbed thereinto. The CO₂-absorbing solution that has absorbed CO₂ issent to a CO₂-regenerator and is decarboxylated with heating by aregeneration heater that regenerates the CO₂-absorbing solution, andthereby, a high-concentration CO₂ gas is recovered. The CO₂-absorbingsolution after the decarboxylation is supplied to the CO₂-absorber by aliquid feed pump, and the CO₂-absorbing solution is circulated and usedbetween the CO₂-absorber and the CO₂-regenerator.

CITATION LIST Patent Literature

[PTL 1] Japanese Patent No. 5237204

SUMMARY

Meanwhile, in a CO₂ recovery unit described in PTL 1, operation isperformed in a state where a constant CO₂ recovery rate is maintained byadjusting the CO₂-absorbing solution to be supplied to the CO₂-absorberand the flow rate of saturated steam to be supplied to a regenerationheater of the CO₂-regenerator, based on the gas flow rate of the fluegas, the introduction temperature of the flue gas, or the like. However,in a case where such control is performed, a control in which the CO₂recovery unit operates in a state where the constant CO₂ recovery rateis maintained even if the CO₂ concentration in the flue gas and the gasflow rate of the flue gas have increased is performed. Thus, forexample, the load of the CO₂ recovery unit may increase if there arerestrictions or the like to the amount of steam used in the CO₂ recoveryunit.

One or more embodiments of the invention provide a CO₂ recovery unit anda CO₂ recovery method that enable stable operation to continue even ifoperation conditions have changed.

A CO₂ recovery unit in accordance with one or more embodiments includesa CO₂-absorber that brings a gas (i.e., gas to be treated) and aCO₂-absorbing solution into contact with each other to cause CO₂included in the gas to be absorbed into the CO₂-absorbing solution; aCO₂-regenerator that heats the CO₂-absorbing solution which has absorbedCO₂, releases CO₂ from the CO₂-absorbing solution, and regenerates theCO₂-absorbing solution; and a CO₂ recovery amount controller(“controller” used interchangeably with “control unit”) that calculatesa computed target value of a CO₂ recovery amount and a computed targetvalue of a CO₂ recovery rate based on a set value of the CO₂ recoveryrate, actual measured values of CO₂ concentration, gas flow rate, andtemperature of the gas, and maximum values of the CO₂ recovery amountsin the CO₂ absorber and the CO₂-regenerator and that controls the amountof the CO₂-absorbing solution supplied to the CO₂-absorber, the amountof the CO₂-absorbing solution supplied to the CO₂-regenerator and theamount of saturated steam supplied to a regeneration heater of theCO₂-regenerator, based on the set value of the CO₂ recovery rate or thecomputed target value of the CO₂ recovery rate.

According to this CO₂ recovery unit, the circulation amount of theCO₂-absorbing solution and the amount of the saturated steam supplied tothe regeneration heater can be appropriately controlled according tochanges in the actual measured values of the CO₂ recovery rate and theCO₂ recovery amount in the gas. Accordingly, even when there is aninfluence on a predetermined relational expression to be used forcontrol and the precision of a measuring instrument due to changes inoperation conditions and the measuring instrument, the CO₂ recovery unitthat can control the CO₂ recovery amount and/or the CO₂ recovery rate toa target value with high precision can be realized.

In the CO₂ recovery unit, the CO₂ recovery amount controller maycalculate the computed target value of the CO₂ recovery rate based onthe following Formulas (1) to (3).Y1=X1×X2×X3×α  Formula (1)Y2=min(X4,Y1)  Formula (2)Y3=Y2/(X2×X3×α)  Formula (3)(In Formulas (1) to (3), X1 represents the set value of the CO₂ recoveryrate, X2 represents an actual measured value of the CO₂ concentration ofthe gas, X3 represents an actual measured value of the gas flow rate ofthe gas, X4 represents a maximum value of the CO₂ recovery amount, Y1represents a target value of the CO₂ recovery amount, Y2 represents acomputed target value of the CO₂ recovery amount, Y3 represents acomputed target value of the CO₂ recovery rate, and a represents aconversion factor).

In the CO₂ recovery unit, the CO₂ recovery amount controller maycalculate the computed target value of the CO₂ recovery rate based onthe maximum value of the CO₂ recovery amount when (i.e., in a casewhere) a target value of the CO₂ recovery amount exceeds the maximumvalue of the CO₂ recovery amount, and may calculate the computed targetvalue of the CO₂ recovery rate based on the calculated target value ofthe CO₂ recovery amount when the target value of the CO₂ recovery amountis equal to or lower than the maximum value of the CO₂ recovery amount.

In the CO₂ recovery unit, the CO₂ recovery amount controller maycalculate the computed target value of the CO₂ recovery amount based onthreshold values when the actual measured values of the gas flow rate,CO₂ concentration, and temperature of the gas exceed predeterminedthreshold values.

In the CO₂ recovery unit, the CO₂ recovery amount controller mayfeedback-control the operation of the overall device using the computedtarget value of the CO₂ recovery amount.

A CO₂ recovery method in accordance with one or more embodimentsincludes a process of bringing a gas (i.e., gas to be treated) and aCO₂-absorbing solution into contact with each other to cause CO₂included in the gas to be absorbed into the CO₂-absorbing solution in aCO₂-absorber; and a process of heating the CO₂-absorbing solution whichhas absorbed CO₂, releasing CO₂ from the CO₂-absorbing solution, andregenerating the CO₂-absorbing solution in a CO₂-regenerator. A computedtarget value of a CO₂ recovery amount and a computed target value of aCO₂ recovery rate are calculated based on a set value of the CO₂recovery rate, actual measured values of CO₂ concentration, gas flowrate, and temperature of the gas, and maximum values of the CO₂ recoveryamounts in the CO₂-absorber and the CO₂-regenerator, and the amount ofthe CO₂-absorbing solution supplied to the CO₂-absorber, the amount ofthe CO₂-absorbing solution supplied to the CO₂-regenerator, and theamount of saturated steam supplied to a regeneration heater of theCO₂-regenerator are controlled based on the set value of the CO₂recovery rate or the computed target value of the CO₂ recovery rate.

According to this CO₂ recovery method, the circulation amount of theCO₂-absorbing solution and the amount of the saturated steam supplied tothe regeneration heater can be appropriately controlled according tochanges in the actual measured values of the CO₂ recovery rate and theCO₂ recovery amount in the gas. Accordingly, even when there is aninfluence on a predetermined relational expression to be used forcontrol and the precision of a measuring instrument due to changes inoperation conditions and the measuring instrument, the CO₂ recoverymethod that can control the CO₂ recovery amount and/or the CO₂ recoveryrate to a target value with high precision can be realized.

In the CO₂ recovery method, the computed target value of the CO₂recovery rate may be calculated based on the following Formulas (1) to(3).Y1=X1×X2×X3×α  Formula (1)Y2=min(X4,Y1)  Formula (2)Y3=Y2/(X2×X3×α)  Formula (3)(In Formulas (1) to (3), X1 represents the set value of the CO₂ recoveryrate, X2 represents an actual measured value of the CO₂ concentration ofthe gas, X3 represents an actual measured value of the gas flow rate ofthe gas, X4 represents a maximum value of the CO₂ recovery amount, Y1represents a target value of the CO₂ recovery amount, Y2 represents acomputed target value of the CO₂ recovery amount, Y3 represents acomputed target value of the CO₂ recovery rate, and a represents aconversion factor).

In the CO₂ recovery method, the computed target value of the CO₂recovery rate may be calculated based on a maximum value of the CO₂recovery amount when a target value of the CO₂ recovery amount exceeds amaximum value of the CO₂ recovery amount, and may be calculated based onthe target value of the CO₂ recovery amount when the target value of theCO₂ recovery amount is equal to or lower than the maximum value of theCO₂ recovery amount.

In the CO₂ recovery method, the computed target value of the CO₂recovery amount may be calculated based on threshold values when theactual measured values of the gas flow rate, CO₂ concentration, andtemperature of the gas exceed predetermined threshold values.

In the CO₂ recovery method, the operation of the overall device may befeedback-controlled using the computed target value of the CO₂ recoveryamount.

Accordingly, a CO₂ recovery unit and a CO₂ recovery method in accordancewith one or more embodiments enable stable operation to continue even ifoperation conditions have changed can be realized.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a CO₂ recovery unit in accordance with oneor more embodiments of the invention.

FIG. 2 is a functional block diagram of a CO₂ recovery amount controllerin accordance with one or more embodiments of the invention.

FIG. 3 is a block diagram illustrating operation control using the CO₂recovery amount controller in accordance with one or more embodiments ofthe invention.

DESCRIPTION OF EMBODIMENTS

The present inventors have noted that, in a related-art CO₂ recoveryunit that operates in a state where a constant CO₂ recovery rate ismaintained, a control in which the CO₂ recovery unit operates in a statewhere the constant CO₂ recovery rate is maintained even if the CO₂concentration in a gas to be treated and the gas flow rate of flue gashave increased may be performed, and the load of the CO₂ recovery unitmay increase. The present inventors also have found out that CO₂recovery amount calculated using a gas flowmeter and a gas concentrationmeter is controlled such that the CO₂ recovery amount becomes equal toor lower than a reference value, so that an increase in the load of theCO₂ recovery unit can be prevented even when operation conditions havefluctuated.

Hereinafter, embodiments of the invention will be described in detailwith reference to the accompanying drawings. However, the invention isnot limited to the following embodiments, and can be appropriatelychanged and carried out. Additionally, the configuration of thefollowing CO₂ recovery unit can be appropriately combined and carriedout.

FIG. 1 is a schematic view of a CO₂ recovery unit in accordance with oneor more embodiments of the invention. As illustrated in FIG. 1, a CO₂recovery unit 1 is an device that absorbs CO₂ in flue gas (an example ofa gas to be treated) 11A containing CO₂ exhausted from industrialfacilities, such as a boiler and a gas turbine, and recovers ahigh-concentration CO₂ gas. The CO₂ recovery unit 1 includes a quencher12 that cools the flue gas 11A containing CO₂ exhausted from industrialfacilities, such as a boiler and a gas turbine; a CO₂-absorber 14 thatis provided in a subsequent stage of the quencher 12, brings the cooledflue gas 11A into contact with a CO₂-absorbing solution 13, and makesthe CO₂-absorbing solution 13 absorb and remove CO₂ in the flue gas 11A;and a CO₂-regenerator 15 that is provided in the subsequent stage of theCO₂-absorber 14, releases CO₂ from the CO₂-absorbing solution 13 thathas absorbed the CO₂, and regenerates the CO₂-absorbing solution 13.

In the CO₂ recovery unit 1, the CO₂-absorbing solution 13 circulatesbetween the CO₂-absorber 14 and the CO₂-regenerator 15. TheCO₂-absorbing solution 13 (lean solution) absorbs CO₂ in theCO₂-absorber 14, and is supplied to the CO₂-regenerator 15 as theCO₂-absorbing solution 13 (rich solution). Additionally, theCO₂-absorbing solution 13 (rich solution) has substantially all CO₂removed and regenerated in the CO₂-regenerator 15, and is supplied tothe CO₂-absorber 14 as the CO₂-absorbing solution 13 (lean solution).

The quencher 12 has a quenching section 121 that cools the flue gas 11A.A circulation line L1 is provided between a bottom part of the quencher12 and a top part of the quenching section 121. A heat exchanger 122that cools cooling water W1, and a circulation pump 123 that circulatethe cooling water W1 within the circulation line L1 are provided in thecirculation line L1.

In the quenching section 121, the flue gas 11A is cooled by bringing theflue gas 11A into countercurrent contact with the cooling water W1. Theheat exchanger 122 cools the cooling water W1 heated by the heatexchange with the flue gas 11A. The circulation pump 123 supplies thecooling water W1, which has flowed down to the bottom part of thequencher 12, to the top part of the quenching section 121 via the heatexchanger 122.

The CO₂-absorber 14 includes a CO₂ absorption section 141 that isprovided on a lower part side of the CO₂-absorber 14 and has the fluegas 11A cooled in the quencher 12 supplied thereto, a washing section142 that is provided on an upper part side of the CO₂-absorber 14. Aliquid storage section 144 that stores cleaning water W2 for cleaning aflue gas 11B from which CO₂ has been removed is provided at a bottompart of the washing section 142. A circulation line L2, through whichthe cleaning water W2, containing the CO₂-absorbing solution 13recovered in the liquid storage section 144, is supplied and circulatedfrom a top part side of the washing section 142, is provided between theliquid storage section 144 and an upper part of the washing section 142.The circulation line L2 is provided with a heat exchanger 21 that coolsthe cleaning water W2, and a circulation pump 22 that circulates thecleaning water W2, containing the CO₂-absorbing solution 13 recovered inthe liquid storage section 144, within the circulation line L2 via theheat exchanger 21. Additionally, the circulation line L2 is providedwith an extraction line L3 through which a portion of the cleaning waterW2 (cleaning water W3) is extracted and supplied to the CO₂ absorptionsection 141. The extraction line L3 is provided with a control valve 23that adjusts the amount of supply of cleaning water W3 supplied to theCO₂-absorbing solution 13 (lean solution).

In the CO₂ absorption section 141, the flue gas 11A containing CO₂ andthe CO₂-absorbing solution 13 containing alkanolamine or the like comeinto countercurrent contact with each other. Accordingly, CO₂ in theflue gas 11A is absorbed by the CO₂-absorbing solution 13 through achemical reaction shown in the following reaction formula. As a result,the flue gas 11A containing CO₂ becomes the flue gas 11B from which CO₂has been removed by passing through the CO₂ absorption section 141.R—NH₂+H₂O+CO₂→R—NH₃HCO₃

In the washing section 142, the flue gas 11B from which CO₂ has beenremoved rises via a chimney tray 145. Then, the flue gas 11B is broughtinto gas-liquid contact with the cleaning water W2 supplied from the toppart side of the washing section 142, and becomes a flue gas 11C fromwhich the CO₂-absorbing solution 13 entrained in the flue gas 11B hasbeen recovered by circulation cleaning. The flue gas 11C is exhausted tothe outside from a tower top part 14 a of the CO₂-absorber 14 after mistin the gas is trapped by a mist eliminator 146.

A rich solution supply tube 50 through which the CO₂-absorbing solution13 (rich solution), which has absorbed CO₂ in the CO₂-absorber 14, issupplied to an upper part side of the CO₂-regenerator 15, is providedbetween a tower bottom part 14 b of the CO₂-absorber 14 and an upperpart of the CO₂-regenerator 15. The rich solution supply tube 50 isprovided with a rich solvent pump 51 that supplies the CO₂-absorbingsolution 13 (rich solution), which has absorbed CO₂ in the CO₂-absorber14, toward the CO₂-regenerator 15, and a rich-lean solution heatexchanger 52 that heats the CO₂-absorbing solution 13 (rich solution)that has absorbed CO₂, using the CO₂-absorbing solution 13 (leansolution) which has been heated with saturated steam and from which CO₂has been removed.

The CO₂-regenerator 15 includes a CO₂-absorbing solution supply part 151that is provided at a central part of the CO₂-regenerator 15 and has theCO₂-absorbing solution 13, which has absorbed CO₂, supplied thereto, anda mirror surface part 152 of a tower bottom part 15 b of a lower part ofthe CO₂-absorbing solution supply part 151. The tower bottom part 15 bof the CO₂-regenerator 15 is provided with a circulation line L4 throughwhich the CO₂-absorbing solution 13 that has flowed down to the towerbottom part 15 b circulates. The circulation line L4 is provided with aregeneration heater 31 that heats the CO₂-absorbing solution 13 withsaturated steam S.

A tower top part 15 a of the CO₂-regenerator 15 is provided with a gasexhaust line L5 through which a CO₂ gas 41 accompanied by the saturatedsteam S is exhausted. The gas exhaust line L5 is provided with acondenser 42 that condenses moisture in the CO₂ gas 41, and a separationdrum 43 that separates the CO₂ gas 41 from condensed water W5. Theseparation drum 43 separates the condensed water W5 from the CO₂ gas 41,and releases a CO₂ gas 44, from which the condensed water W5 has beenseparated, from an upper part of the separation drum 43 to the outside.A condensed water line L6 through which the condensed water W5 separatedby the separation drum 43 is supplied to the upper part of theCO₂-regenerator 15 is provided between a bottom part of the separationdrum 43 and the upper part of the CO₂-regenerator 15. The condensedwater line L6 is provided with a condensed water circulation pump 45that supplies the condensed water W5 separated by the separation drum 43to the upper part of the CO₂-regenerator 15.

Additionally, the tower bottom part 15 b of the CO₂-regenerator 15 andan upper part of the CO₂ absorption section 141 of the CO₂-absorber 14are provided with a lean solution supply tube 53 through which theCO₂-absorbing solution 13 (lean solution) in the tower bottom part 15 bof the CO₂-regenerator 15 is supplied to the upper part of the CO₂absorption section 141. The lean solution supply tube 53 is providedwith the rich-lean solution heat exchanger 52 that heats theCO₂-absorbing solution 13 (rich solution), which has absorbed CO₂, usingthe CO₂-absorbing solution 13 (lean solution) which has been heated withthe saturated steam S and from which CO₂ has been removed, a leansolution pump 54 that supplies the CO₂-absorbing solution 13 (leansolution) in the tower bottom part 15 b of the CO₂-regenerator 15 to theupper part of the CO₂ absorption section 141, and a quenching section 55that cools the CO₂-absorbing solution 13 (lean solution) to apredetermined temperature.

The CO₂ recovery unit 1 in accordance with one or more embodimentsincludes a flue gas detecting unit 101 that is provided in a flowpassage for the flue gas 11A to be introduced into the quencher 12, aCO₂ concentration meter 102 that is provided in a flow passage for theflue gas 11C exhausted from CO₂-absorber 14, a CO₂ gas detecting unit103 that is provided in a flow passage for the CO₂ gas 44 exhausted fromthe separation drum 43, and a concentration meter 104 that measures theconcentration of the CO₂-absorbing solution (lean solution) 13 to besupplied to the CO₂-absorber 14.

A flue gas detecting unit 101 a measures CO₂ concentration in the fluegas 11A to be introduced into the quencher 12, and transmits themeasured CO₂ concentration to a CO₂ recovery amount controller 111.Additionally, a flue gas detecting unit 101 b measures the gas flow rateand the gas temperature of the flue gas 11A, and transmits the gas flowrate and the gas temperature to the CO₂ recovery amount controller 111.The CO₂ concentration meter 102 detects CO₂ concentration in the fluegas 11C exhausted from the CO₂-absorber 14, and transmits the detectedCO₂ concentration to the CO₂ recovery amount controller 111.

The CO₂ gas detecting unit 103 detects the gas flow rate and theconcentration of the CO₂ gas 44 exhausted from the separation drum 43,and transmits the gas flow rate and the concentration to the CO₂recovery amount controller 111. The concentration meter 104 measures theconcentration of the CO₂-absorbing solution (lean solution) 13 to besupplied to the CO₂-absorber 14, and transmits the measuredconcentration of the CO₂-absorbing solution (lean solution) 13 to theCO₂ recovery amount controller 111.

The CO₂ recovery amount controller 111 calculates a computed targetvalue of the CO₂ recovery rate based on a set value of the CO₂ recoveryrate, actual measured values of the CO₂ concentration, gas flow rate,and temperature of the flue gas 11A, and a maximum value of the CO₂recovery amount of the overall device. Additionally, the CO₂ recoveryamount controller 111 controls the amount of the CO₂-absorbing solution13 supplied to the CO₂-absorber 14 and the amount of the CO₂-absorbingsolution 13 supplied to the CO₂-regenerator 15 based on the calculatedcomputed target value of the CO₂ recovery rate, and controls the amountof the saturated steam S supplied to the regeneration heater 31 of theCO₂-regenerator 15.

FIG. 2 is a functional block diagram of the CO₂ recovery amountcontroller 111 in accordance with one or more embodiments of theinvention. The CO₂ recovery amount controller 111 in accordance with oneor more embodiments includes a calculating unit 112, and a flue gascontroller 113, an absorbing solution controller 114, and a steamcontroller 115. The calculating unit 112 calculates the maximum value ofthe CO₂ recovery amount that is determined according to device designconditions and utility conditions of steam or the like and is capablebeing recovered by the CO₂ recovery unit 1. Additionally, thecalculating unit 112 calculates a target value of the CO₂ recoveryamount based on the set value of the CO₂ recovery rate that is set inadvance, the CO₂ concentration in the flue gas 11A measured by the fluegas detecting unit 101 a, and an actual measured value of the gas flowrate of the flue gas 11A measured by the flue gas detecting unit 101 b.The target value of the CO₂ recovery amount is a CO₂ recovery amountthat is determined based on the set value of the CO₂ recovery rate thatis set in advance. Moreover, the calculating unit 112 calculates thecomputed target value of the CO₂ recovery amount based on the targetvalue of the CO₂ recovery amount and the calculated maximum value of theCO₂ recovery amount. The computed target value of the CO₂ recoveryamount is, for example, any smaller value of the maximum value of theCO₂ recovery amount or and the target value of the CO₂ recovery amount.Additionally, the calculating unit 112 calculates the computed targetvalue of the CO₂ recovery rate based on the calculated computed targetvalue of the CO₂ recovery amount, the CO₂ concentration in the flue gas11A, and the actual measured value of the gas flow rate of the flue gas11A. The computed target value of the CO₂ recovery rate is a value ofthe CO₂ recovery rate for realizing a CO₂ recovery amount according toactual operation conditions or the like of the CO₂ recovery unit 1.Moreover, the calculating unit 112 transmits the calculated computedtarget values of the CO₂ recovery rate and the CO₂ recovery amount tothe flue gas controller 113, the absorbing solution controller 114, andthe steam controller 115.

The calculating unit 112 calculates the computed target value that isthe target value of the CO₂ recovery rate of the overall device based onvarious kinds of input data and various actual measured values. In oneor more embodiments, the calculating unit 112 calculates the computedtarget value of the CO₂ recovery rate, for example, based on thefollowing Formulas (1) to (3). In addition, □ is any value that isdetermined depending on design conditions or the like of the CO₂recovery unit.Y1=X1×X2×X3×α  Formula (1)Y2=min(X4,Y1)  Formula (2)Y3=Y2/(X2×X3×α)  Formula (3)(In Formulas (1) to (3), X1 represents the set value of the CO₂ recoveryrate, X2 represents the actual measured value of the CO₂ concentrationof the flue gas, X3 represents the actual measured value of the gas flowrate of the flue gas, X4 represents the maximum value of the CO₂recovery amount, Y1 represents the target value of the CO₂ recoveryamount, Y2 represents the computed target value of the CO₂ recoveryamount, Y3 represents the computed target value of the CO₂ recoveryrate, and □ represents a conversion factor).

The flue gas controller 113 controls the flow rate of the flue gas 11Ato be introduced into the quencher 12 via a control valve V1 based onthe set value and the computed target value of the CO₂ recovery ratecalculated by the calculating unit 112. The absorbing solutioncontroller 114 controls the liquid volume of the CO₂-absorbing solution(lean solution) 13 to be supplied to the CO₂-absorber 14 via a controlvalve V2 based on the set value and the computed target value of the CO₂recovery rate calculated by the calculating unit 112, and controls theliquid volume of the CO₂-absorbing solution (rich solution) 13 to besupplied to the CO₂-regenerator 15 via a control valve V3. The steamcontroller 115 controls the flow rate of the saturated steam S to besupplied to the regeneration heater 31 via a control valve V4 based onthe set value and the computed target value of the CO₂ recovery ratecalculated by the calculating unit 112.

In one or more embodiments, the calculating unit 112 calculates thecomputed target value of the CO₂ recovery rate based on the maximumvalue of the CO₂ recovery amount when the target value of the CO₂recovery amount exceeds the maximum value of the CO₂ recovery amount,and calculates the computed target value of the CO₂ recovery rate basedon the target value of the CO₂ recovery amount when the target value ofthe CO₂ recovery amount is equal to or lower than the maximum value ofthe CO₂ recovery amount. Accordingly, even when operation conditions,such as the CO₂ concentration, gas flow rate, and temperature of theflue gas 11A, fluctuate, and CO₂ introduced into CO₂ recovery unit 1 hasincreased, the CO₂ recovery unit 1 can be stably operated because theoverall device can be controlled based on the set value of the CO₂recovery rate at which the CO₂ recovery amount becomes equal to or lowerthan a maximum value capable of being processed by the overall device.Additionally, when CO₂ introduced into the CO₂ recovery unit 1 becomesequal to or lower than the maximum value of the CO₂ recovery amount, theoverall device can be controlled based on the set value of the CO₂recovery rate that is set in advance. Thus, an operational stateaccording to a design can be brought about.

Next, the overall operation of the CO₂ recovery unit 1 in accordancewith one or more embodiments will be described. The flue gas 11Acontaining CO₂ exhausted from industrial facilities, such as a boilerand a gas turbine, is introduced into the quencher 12, and is broughtinto countercurrent contact with and cooled by the cooling water W1after the CO₂ concentration, gas flow rate, and temperature in the fluegas 11A are measured by the flue gas detecting unit 101. The cooled fluegas 11A is introduced into the CO₂-absorber 14 via a flue 16. The fluegas 11A introduced into the CO₂-absorber 14 is brought intocountercurrent contact with the CO₂-absorbing solution 13 containingalkanolamine or the like in the CO₂ absorption section 141, and becomesthe flue gas 11B from which CO₂ in the flue gas 11A has been absorbed bythe CO₂-absorbing solution 13 and CO₂ has been removed.

The flue gas 11B from which CO₂ has been removed rises via the chimneytray 145, is brought into gas-liquid contact with the cleaning water W2supplied from the top part side of the washing section 142, and becomesthe flue gas 11C from which the CO₂-absorbing solution 13 entrained inthe flue gas 11B has been recovered by circulation cleaning. The CO₂concentration in the flue gas 11C is measured by the CO₂ concentrationmeter 102 and the flue gas 11C is exhausted from the tower top part 14 aof the CO₂-absorber 14 to the outside, after the mist in the gas iscaught by the mist eliminator 146.

The CO₂-absorbing solution 13 (rich solution) that has absorbed CO₂ issent to the rich-lean solution heat exchanger 52 by a rich solvent pump51 via a rich solution supply tube 50 in the CO₂-absorber 14. In therich-lean solution heat exchanger 52, the CO₂-absorbing solution 13(rich solution) sent from the CO₂-absorber 14 is heat-exchanged with theCO₂-absorbing solution 13 (lean solution) sent from the CO₂-regenerator15. The CO₂-absorbing solution 13 (rich solution) after this heatexchange is supplied to the upper part of the CO₂-regenerator 15. TheCO₂-absorbing solution 13 supplied to the CO₂-regenerator 15 has CO₂removed therefrom and becomes a semi-lean solution, while flowing downto the tower bottom part 15 b via the CO₂-absorbing solution supply part151. This semi-lean solution is circulated through the circulation lineL4, is heated by the saturated steam S in the regeneration heater 31,and becomes the CO₂-absorbing solution 13 (lean solution). The saturatedsteam S after being heated becomes the saturated steam condensed waterW4. The CO₂ gas 41 removed from the CO₂-absorbing solution 13 isreleased to the outside as the CO₂ gas 44 from which the condensed waterW5 has been separated through the upper part of the separation drum 43after the moisture thereof is condensed by the condenser 42. As for theCO₂ gas 44, CO₂ concentration in the CO₂ gas 44 is measured by the CO₂gas detecting unit 103.

The CO₂-absorbing solution 13 (lean solution) of the tower bottom part15 b of the CO₂-regenerator 15 is supplied to the upper part of the CO₂absorption section 141 of the CO₂-absorber 14 by the lean solution pump54 after being heat-exchanged with the CO₂-absorbing solution 13 (richsolution) by the rich-lean solution heat exchanger 52 via the leansolution supply tube 53.

FIG. 3 is a flow chart illustrating operation control using the CO₂recovery amount controller 111 in accordance with one or moreembodiments. As illustrated in FIG. 3, the CO₂ recovery amountcontroller 111 calculates the target value of the CO₂ recovery amountbased on various kinds of input data, such as the CO₂ set value that isset in advance, the CO₂ concentration in the flue gas 11A, and the flowrate and temperature of the flue gas 11A (Step ST1), and determineswhether or not the calculated target value of the CO₂ recovery amount isequal to or lower than the maximum value of the CO₂ recovery amount(Step ST2). Then, when the target value of the CO₂ recovery amount isequal to or lower than the maximum value of the CO₂ recovery amount, theCO₂ recovery amount controller 111 controls the flow rate of theCO₂-absorbing solution 13 and the flow rate of the saturated steam S tobe supplied to the regeneration heater 31 via the absorbing solutioncontroller 114 and the steam controller 115, based on the set value ofthe CO₂ recovery rate (Step ST3A). Additionally, when the target valueof the CO₂ recovery amount exceeds the maximum value of the CO₂ recoveryamount, the CO₂ recovery amount controller 111 controls the flow rate ofthe CO₂-absorbing solution 13 and the flow rate of the saturated steam Sto be supplied to the regeneration heater 31 via the absorbing solutioncontroller 114 and the steam controller 115, based on the computedtarget value of the CO₂ recovery rate (Step ST3B).

As described above, according to one or more embodiments, thecirculation amount of the CO₂-absorbing solution 13 and the amount ofthe saturated steam S supplied to the regeneration heater 31 can beappropriately controlled according to changes in the target values ofthe CO₂ recovery rate and the CO₂ recovery amount in the flue gas 11A.Accordingly, for example, even when there are changes in operationconditions, such as a case where CO₂ concentration in flue gas hasincreased, or even when there is an influence on a predeterminedrelational expression to be used for operation control and the precisionof a measuring instrument due to change of the measuring instrument, orthe like, the CO₂ recovery unit 1 that can control the CO₂ recoveryamount and/or the CO₂ recovery rate to a target value with highprecision can be realized.

In addition, in the above-described embodiments, the CO₂ recovery amountcontroller 111 may continuously monitor the gas flow rate, CO₂concentration, and temperature of the flue gas 11A, thereby calculatingthe computed target value of the CO₂ recovery rate. However, the presentinvention is not limited to this configuration. For example, the CO₂recovery amount controller 111 may control the CO₂ recovery amount to avalue equal to or lower than the computed target value of the CO₂recovery amount when the actual measured values of the gas flow rate,CO₂ concentration, and temperature of the flue gas 11A exceedpredetermined threshold values. By performing such control, for example,even when a malfunction has occurred in measuring instruments, such as aCO₂ concentration meter and a gas flowmeter, it is possible to operatethe CO₂ recovery unit 1 appropriately.

Additionally, in the above-described embodiments, the CO₂ recoveryamount controller 111 may feedback-control the operation of the overalldevice using the computed target value of the CO₂ recovery amount. Byperforming such control, the operation of the CO₂ recovery unit 1 can becontrolled based on the CO₂ recovery amount in which the response ofnumerical fluctuations is quick relative to the CO₂ recovery rate. Thus,it is possible to control the operation of the CO₂ recovery unit 1 morestably.

In addition, in the above-described embodiments, the flue gas 11Acontaining CO₂ exhausted from industrial facilities, such as a boilerand a gas turbine, is treated by the CO₂-absorbing solution 13. However,the gas to be treated is not limited to the flue gas 11A; in otherembodiments of the invention, various other gases containing CO₂ may betreated by the CO₂-absorbing solution 13.

REFERENCE SIGNS LIST

-   1: CO₂ RECOVERY UNIT-   11A, 11B, 11C: FLUE GAS-   12: QUENCHER-   121: QUENCHING SECTION-   122: HEAT EXCHANGER-   123: CIRCULATION PUMP-   13: CO₂-absorbing solution-   14: CO₂-ABSORBER-   14 a: TOWER TOP PART-   14 b: TOWER BOTTOM PART-   141: CO₂ ABSORPTION SECTION-   142: WASHING SECTION-   144: LIQUID STORAGE SECTION-   145: CHIMNEY TRAY-   146: MIST ELIMINATOR-   15: CO₂-regenerator-   15 a: TOWER TOP PART-   15 b: TOWER BOTTOM. PART-   151: CO₂-absorbing solution SUPPLY PART-   152: MIRROR SURFACE PART-   16: FLUE-   21: HEAT EXCHANGER-   22: CIRCULATION PUMP-   23: CONTROL VALVE-   31: REGENERATION HEATER-   41, 44: CO₂ GAS-   42: CONDENSER-   43: SEPARATION DRUM-   45: CONDENSED WATER CIRCULATION PUMP-   50: RICH SOLUTION SUPPLY TUBE-   51: RICH SOLVENT PUMP-   52: RICH-LEAN SOLUTION HEAT EXCHANGER-   53: LEAN SOLUTION SUPPLY TUBE-   54: LEAN SOLUTION PUMP-   55: QUENCHING SECTION-   101 a, 101 b: FLUE GAS DETECTING UNIT-   102: CO₂ CONCENTRATION METER-   103: CO₂ GAS DETECTING UNIT-   104: CONCENTRATION METER-   111: CO₂ RECOVERY AMOUNT CONTROLLER-   112: CALCULATING UNIT-   113: FLUE GAS CONTROLLER-   114: ABSORBING SOLUTION CONTROLLER-   115: STEAM CONTROLLER-   L1, L2, L4: CIRCULATION LINE-   L3: EXTRACTION LINE-   L5: GAS EXHAUST LINE-   L6: CONDENSED WATER LINE-   S: SATURATED STEAM-   W1: COOLING WATER-   W2, W3: CLEANING WATER-   W4: SATURATED STEAM CONDENSED WATER-   W5: CONDENSED WATER

While the disclosure includes a limited number of embodiments, thoseskilled in the art, having benefit of this disclosure, will appreciatethat other embodiments may be devised which do not depart from the scopeof the present disclosure. Accordingly, the scope should be limited onlyby the attached claims. Further, one of ordinary skill in the art wouldappreciate that the various “units” disclosed herein may be implementedby software or hardware (e.g., circuit).

What is claimed:
 1. A CO₂ recovery unit comprising: a CO₂-absorber inwhich a gas containing CO₂ contacts a CO₂-absorbing solution thatabsorbs the CO₂ in the gas; a CO₂-regenerator that heats theCO₂-absorbing solution that has absorbed the CO₂ from the gas, releasesthe CO₂ from the CO₂-absorbing solution, and regenerates theCO₂-absorbing solution; and a CO₂ recovery amount controller that:calculates a computed target value of a CO₂ recovery amount and acomputed target value of a CO₂ recovery rate based on a set value of theCO₂ recovery rate, actual measured values of CO₂ concentration, gas flowrate, and temperature of the gas, calculates a maximum value of the CO₂recovery amount in the CO₂-absorber and a maximum value of the CO₂recovery amount in the CO₂-regenerator, and controls an amount of theCO₂-absorbing solution supplied to the CO₂-absorber, an amount of theCO₂-absorbing solution supplied to the CO₂-regenerator, and an amount ofsaturated steam supplied to a regeneration heater of the CO₂-regeneratorbased on the set value of the CO₂ recovery rate or the computed targetvalue of the CO₂ recovery rate, wherein the CO₂ recovery amountcontroller controls the CO₂ recovery amount to a value less than orequal to the computed target value of the CO₂ recovery amount when anyone of the actual measured values of the CO₂ concentration, gas flowrate, and temperature of the gas exceeds a predetermined thresholdvalue.
 2. The CO₂ recovery unit according to claim 1, wherein the CO₂recovery amount controller calculates the computed target value of theCO₂ recovery rate based on the following formulas (1) to (3):Y1=X1×X2×X3×α  Formula (1)Y2=min(X4,Y1)  Formula (2)Y3=Y2/(X2×X3×α)  Formula (3) wherein, X1 represents the set value of theCO₂ recovery rate, X2 represents an actual measured value of the CO₂concentration of the gas, X3 represents an actual measured value of thegas flow rate of the gas, X4 represents a maximum value of the CO₂recovery amount, Y1 represents a target value of the CO₂ recoveryamount, Y2 represents a computed target value of the CO₂ recoveryamount, Y3 represents a computed target value of the CO₂ recovery rate,and a represents a conversion factor.
 3. The CO₂ recovery unit accordingto claim 1, wherein the CO₂ recovery amount controller: calculates thecomputed target value of the CO₂ recovery rate based on the maximumvalue of the CO₂ recovery amount when a target value of the CO₂ recoveryamount exceeds a maximum value of the CO₂ recovery amount, andcalculates the computed target value of the CO₂ recovery rate based onthe calculated target value of the CO₂ recovery amount when the targetvalue of the CO₂ recovery amount is less than or equal to the maximumvalue of the CO₂ recovery amount.
 4. The CO₂ recovery unit according toclaim 1, wherein the CO₂ recovery amount controller feedback-controls anoperation of the CO₂ recovery unit using the computed target value ofthe CO₂ recovery amount.
 5. A CO₂ recovery method comprising: causing agas containing CO₂ to contact a CO₂-absorbing solution that absorbs theCO₂ in the gas in a CO₂-absorber; heating the CO₂-absorbing solutionthat has absorbed CO₂ from the gas, releasing the CO₂ from theCO₂-absorbing solution, and regenerating the CO₂-absorbing solution in aCO₂-regenerator; calculating a computed target value of a CO₂ recoveryamount and a computed target value of a CO₂ recovery rate based on a setvalue of the CO₂ recovery rate, actual measured values of CO₂concentration, gas flow rate, and temperature of the gas with a CO₂recovery amount controller; calculating a maximum value of the CO₂recovery amount in the CO₂-absorber and a maximum value of the CO₂recovery amount in the CO₂-regenerator with the CO₂ recovery amountcontroller; and controlling an amount of the CO₂-absorbing solutionsupplied to the CO₂-absorber, an amount of the CO₂-absorbing solutionsupplied to the CO₂ regenerator, and an amount of saturated steamsupplied to a regeneration heater of the CO₂-regenerator based on theset value of the CO₂ recovery rate or the computed target value of theCO₂ recovery rate with the CO₂ recovery amount controller; and whereinthe CO₂ recovery amount is controlled to a value less than or equal tothe computed target value of the CO₂ recovery amount when any one of theactual measured values of the CO₂ concentration, gas flow rate, andtemperature of the gas exceeds a predetermined threshold value.
 6. TheCO₂ recovery method according to claim 5, wherein the computed targetvalue of the CO₂ recovery rate is calculated based on the followingformulas (1) to (3):Y1=X1×X2×X3×α  Formula (1)Y2=min(X4,Y1)  Formula (2)Y3=Y2/(X2×X3×α)  Formula (3) wherein, X1 represents the set value of theCO₂ recovery rate, X2 represents an actual measured value of the CO₂concentration of the gas, X3 represents an actual measured value of thegas flow rate of the gas to be treated, X4 represents a maximum value ofthe CO₂ recovery amount, Y1 represents a target value of the CO₂recovery amount, Y2 represents a computed target value of the CO₂recovery amount, Y3 represents a computed target value of the CO₂recovery rate, and a represents a conversion factor.
 7. The CO₂ recoverymethod according to claim 5, wherein the computed target value of theCO₂ recovery rate is calculated based on a maximum value of the CO₂recovery amount when a target value of the CO₂ recovery amount exceeds amaximum value of the CO₂ recovery amount; and the computed target valueof the CO₂ recovery rate is calculated based on the target value of theCO₂ recovery amount when the target value of the CO₂ recovery amount isless than or equal to the maximum value of the CO₂ recovery amount. 8.The CO₂ recovery method according to claim 5, wherein the method isperformed by a CO₂ recovery unit and the method further comprisesfeedback-controlling an operation of the CO₂ recovery unit using thecomputed target value of the CO₂ recovery amount.